The present invention relates to a method of maintaining the inside surfaces of a heat exchanger substantially clean with respect to incrustations deposited from an influent fluid in indirect heat exchange relation with another fluid. The invention is particularly applicable to a brine heat exchanger in which feed brine for an electrolytic cell is preheated by incoming hot acidic chlorine water. From the incoming brine is deposited a calcium carbonate fouling layer on the brine side of the heat exchanger, which can be removed by reaction with the acidic chlorine water in accordance with the teachings of this invention.
It is known to contact hot, humid chlorine gas emanating from an electrolytic cell with feed brine for the cell in a heat exchanger whereby heat is transferred to the cell feed brine. U.S. Pat. No. 3,434,948, for example, shows a method for utilizing the sensible heat of cell gases to heat electrolyte feed solutions. Hot hydrogen cell gas is passed through a heat exchanger which transfers heat from the gas to cool feed brine. In another commercial embodiment, hot chlorine cell gas is passed into a direct contact cooler, and the hot effluent water is passed into a heat exchanger within which heat is transferred to cold influent cell feed brine. It has been found, however, in the operation of a heat exchanger of this type, that a calcium carbonate fouling layer is deposited on the brine side of the heat exchanger. These incrustations cause restriction of the flow of fluid through the heat exchanger and require disconnecting the brine process piping in order to introduce an acid cleaning solution, such as inhibited muriatic acid. A similar method has been suggested by Natwick in U.S. Pat. No. 2,328,837, where a strong aqueous sulfur dioxide solution is used to dissolve scale deposits in a heat exchanger. This requires not only a time-consuming shut-down of the process equipment, with an additional demand on the time of maintenance personnel, but also requires the introduction of special cleaning solutions, which result in additional cost.
In U.S. Pat. No. 920,570 to Heintzelman et al. is disclosed a method for cleaning the oil side of a heat exchanger used for preheating oil by the heat transferred from steam. A valve piping connection allows discharge of steam into the oil side, allowing obstructions which might have accumulated on the oil side to be forced from the heat exchanger under the direct blast of steam. No disclosure, however, of an adaptation of this concept to chemical, rather than mechanical, cleaning of an opposite side of a heat exchanger can be found in Heintzelman et al. Use of steam to accomplish the cleaning of incrustations deposited from the solution of a liquid-liquid heat exchanger is neither practical nor economical, and would require an even more complicated interruption in service of process equipment than that which would result from using the process of the Natwick patent.
U.S. Pat. No. 3,674,687, to Matheson cleans accumulated sludge from a sewage system heat exchanger through contact of the hot scale material with a mixture of cold air and water, which is forced through at high pressure to cause the scale material to contract rapidly and flake away. Reliance upon this mechanical washing action requires addition of a cleaning fluid from an external source, and the effect can be expected to last only as long as a temperature differential between incrustations to be cleaned, and cleaning fluid, is maintained. A purely chemical cleaning action, on the other hand, can be expected to remain effective as long as is necessary to remove incrustations. The present invention is applicable to heat exchangers designed for use with electrolytic cells and hydrochloric acid cells are typical examples of such cells. Since the electrolyte used in such cells is most advantageously maintained at a temperature above the ambient temperature, cold feed brine is often heated before introduction into such cells. Heat exchangers are employed to transfer heat from products of such cells to feed brine. Once such source of heated product is hot chlorine water, formed by dissolution of evolved chlorine gas to form a hot chlorine-water solution. However, this invention is not to be understood as being limited only to heat exchangers adapted for use in conjunction with chlor-alkali cells and hydrochloric acid cells, in that it is applicable to all heat exchangers. Although the invention will not be used only with chlor-alkali cells, the description herein will specifically describe the invention in relationship to a chlor-alkali cell, but it is to be understood that the present invention is also applicable generally to any use of a heat exchanger.